Empty string not being converted to null when passing JSON object to controller

Question

In ASP.NET Core 2.1, I'm noticing basically the opposite is happening to this question: string.empty converted to null when passing JSON object to MVC Controller

When I send a JSON object back to a controller, that has properties with empty strings, they are no longer automatically being converted to null.

For example, take this JS object:

 var person = {firstName:"", lastName:""}; 

        $http.post("/app/index", person)
            .then(function (response) {
                // Success
            }, function (error) {
                // Failure
            })

When bound to the model in the controller, the properties with empty strings are remaining as empty strings:

    //
    // POST: /App
    [HttpPost]
    public async Task<ActionResult> Index([FromBody] AppFormDTO data)
    {
       ....
    }

Which when saving this object to the database, I'd prefer to have null values instead of a bunch of empty strings.

Did they change the DefaultModelBinder in ASP.NET Core 2.1? And if so, how do I change it back to how it was in MVC5 -- where string.empty was automatically converted to null?

UPDATE, to add my AppFormDTO model, for reference:

public class AppFormDTO
{
    public string firstName{ get; set; }
    public string lastName{ get; set; }
}

Answer 1

Add this to your PERSON model for firstName/lastName:

[DisplayFormat(ConvertEmptyStringToNull = false)]
... the model values 

Answer 2

Following the source code of ASP.NET Core

 if (bindingContext.ModelType == typeof(string))
 {
      // Already have a string. No further conversion required but handle ConvertEmptyStringToNull.
      if (bindingContext.ModelMetadata.ConvertEmptyStringToNull && string.IsNullOrWhiteSpace(value))
      {
           model = null;
      }
      else
      {
           model = value;
      }
 }

The behavior by default is set to empty string to null. Recheck your model AppFormDTO if you putted ConvertEmptyStringToNull = false which is not correct.